This invention relates to semiconductor memory devices, and more particularly to an improved sense amplifier for an N-channel MOS memory device of the type employing one-transistor memory cells.
MOS random access memory (RAM) devices are becoming more widely used in the manufacture of digital equipment, particularly minicomputers, as the speed and cost advantages of these devices increase. The cost per bit of storage using MOS RAMs has gone down as the number of bits or memory cells per package goes up. A RAM containing 4096 bits, for example, is shown in U.S. Pat. No. 3,940,747 issued Feb. 24, 1976 to Kuo and Kitagawa, assigned to Texas Instruments, 16,384 bit or "16K" RAMs are described in articles in Electronics, Feb. 19, 1976, pp. 116-121, and May 13, 1976, pp. 81-86, and 64K RAMs will soon be available. As the number of bits increases, the cell size decreases, the magnitude of the storage capacitor in each cell of necessity decreases, and the capacitance of the digit lines increase. These factors reduce the magnitude of the data signal which exists on a digit line. A full logic level, i.e., the difference between a "1" and a "0" , in one of these devices may be perhaps 5 v.; however, the difference in voltage between a "1" and a "0" for the data coupled to a digit line in the memory array from the selected one-transistor cell may be only fifty millivolts. Prior circuits for sensing these low-level signals are shown in U.S. Pat. No. 3,940,747 and the Electronics, Sept. 13, 1973, Vol. 46, No. 19, pp. 116-121, and IEEE Journal of Solid State Circuits, Oct., 1972, p. 336, by Stein et al, as well as U.S. patent application Ser. No. 682,687 filed May 3, 1976 by Kitagawa and McAlexander, and U.S. Pat. No. 4,081,701, issued Apr. 28, 1978, to White, McAdams and Redwine, assigned to Texas Instruments.
As memory devices require higher packing density, higher speed, and lower power dissipation, the sense amplifiers become more critical. Some prior circuits exhibit high power dissipation and overly long charging times for the digit lines, while others require high instantaneous current and critical clock timing. One of the problems has been in exactly matching the two cross-coupled driver transistors; the threshold can best be matched in long channel transistors; but this causes excessive discharge time for the zero-going side of the column line.
It is a principal object of this invention to provide an improved sense amplifier for an MOS RAM, and in particular a sense amplifier which is of high speed operation, as well as high sensitivity.